The present invention relates generally to radiation concentrators and more particularly to point focus radiation concentrators using crossed linear Fresnel lens. The present invention relates even more particularly to radiation concentrators utilized for concentrating solar radiation.
Fresnel lens (echelon lens) are well known and are known to be useful to replace conventional lenses for focusing or refracting radiation. Fresnel lenses can be either annular echelon lenses or linear echelon lenses.
Annular echelon lenses (surfaces) can be used quite well to point focus radiation. Such an annular echelon surface has a high focusing accuracy; however, the size of annular echelon surfaces are limited. Due to the necessity of having a machine, a ruling engine, to cut the master surface as large as the surface being produced, in order to have a three-unit diameter annular echelon lens, it would be necessary to have a ruling engine capable of cutting the master having a size of at least three units in diameter. Since the maximum size of the ruling engine is practically limited, the size of the resulting annular echelon surface is also limited.
Normally a linear echelon surface is used for line focusing. When two linear echelon surfaces are crossed (oriented) at 90 degrees with respect to each other, the combination approximates an annular echelon lens. Incident radiation along the orthogonal meridians of each linear echelon surface can be made to converge quite well to a point focus. The advantage in using linear echelon surfaces is that very large concentrating arrays can be formed by the matrix sectioning of the radiation concentrator into individual linear echelon surfaces. This matrix sectioning is shown by U.S. Pat. No. 4,108,540, Anderson et al, entitled Refractor-Reflector Radiation Concentrator, assigned to the same assignee as the present invention.
Crossed linear echelon surfaces, however, suffer the disadvantage that while radiation incident along the orthogonal meridians converge, radiation incident away from these meridians, for example, radiation incident at a corner of the crossed linear echelon surfaces, will not converge. In fact, this focusing deviation is at its maximum for radiation incident at a corner of the crossed linear echelon surfaces. The inability of crossed linear echelon surfaces to focus all incident radiation to a point is called "crossed-cylinder aberration." This crossed-cylinder aberration is illustrated in U.S. Pat. No. 4,118,114, Anderson et al, entitled Improved Low-Glare Overhead Projector, also assigned to the assignee of the present invention. Thus, while radiation concentrators using crossed linear echelon surfaces can be made very large, the efficiency of a given array is limited to a value substantially less than that possible without any crossed cylinder aberration.
The efficiency of radiation concentrators, and in particular, solar concentrators, can be judged by measuring the radius around the point of focus of the radiation concentrator where 95% of all incident radiation will be focussed. This radius is known as the 95% energy containment radius, R.sub.95. The smaller the R.sub.95, the better and more efficient the radiation concentrator. Another number utilized to judge the efficiency of radiation concentrators is a number known as the geometric concentration ratio, GCR. The geometric concentration ratio may be calculated from the 95% energy containment radius by multiplying the length and width of the radiation concentrator and dividing that by pi times the 95% energy containment radius squared. In equation form, the geometric concentration ratio can be calculated as follows: ##EQU1##
Traditional radiation concentrators utilizing crossed linear echelon surfaces typically have a geometric concentration ratio of less than 100. For a radiation concentrator having a length and a width of approximately 12 units and a focal distance of approximately 12 units, the R.sub.95 value is typically on the order of or greater than one unit.
There is desired a point focus radiation concentrator, particularly for solar radiation, which provides the focusing accuracy (efficiency) of an annular echelon lens with the large size capability of a lens using crossed linear echelon surfaces.